Insulated Glass U-Factor & SHGC Calculator
Determine center-of-glass U-factor, thermal resistance (R-value), Solar Heat Gain Coefficient (SHGC), and condensation resistance for double and triple pane IGUs.
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Insulated Glass Unit (IGU) Physics: U-Factor & Solar Heat Gain
Windows and curtain wall glass represent the most thermally vulnerable component of the modern building envelope. Heat is transferred through an IGU via three coupled thermodynamic mechanisms: conduction through the glass panes and perimeter spacer, convection currents circulating inside the hermetic gas cavity, and long-wave infrared thermal radiation bouncing between facing glass surfaces.
Thermal Performance Metrics & Governing Principles
- Center-of-Glass U-Factor (BTU / hr·ft²·°F):
The rate of non-solar heat transfer through one square foot of glass for every 1°F temperature difference between indoors and outdoors. Lower U-factor indicates superior thermal insulation.R-Value = 1 / U-Factor - Low-Emissivity (Low-E) Microscopic Metallic Coatings:
Standard clear float glass has an emissivity ofε = 0.84, meaning it radiates 84% of absorbed radiant heat. Sputtered microscopic silver coatings (soft-coat Low-E) lower emissivity toε = 0.02 - 0.04, reflecting infrared heat back inside during winter and repelling solar infrared during summer. - Argon & Krypton Gas Infill:
Argon has 33% lower thermal conductivity and higher molecular mass than atmospheric air, suppressing internal convective circulation loops inside 1/2" (12.7 mm) cavities. - Solar Heat Gain Coefficient (SHGC):
The fraction of incident solar radiation admitted through the glass, ranging from 0.0 to 1.0. Modern solar-control low-E glass reflects invisible near-infrared heat while admitting visible light, resulting in a high Light-to-Solar Gain (LSG) ratio:LSG = VLT / SHGC.
Frequently Asked Questions
What is the difference between center-of-glass U-factor and whole-window U-factor?
Center-of-Glass (COG) measures heat transfer through the undisturbed central vision glass. Whole-window U-factor (NFRC certified) includes the thermal bridge of the aluminum/wood/PVC perimeter frame and the edge-of-glass spacer bar, making whole-window U-factor typically 20% to 40% worse (higher) than COG.
Which glass surface should the Low-E coating be placed on?
Surfaces are numbered from exterior (#1) to interior (#4 on a double pane). In heating-dominated northern climates, Low-E is placed on Surface #3 to trap indoor radiant heat. In cooling-dominated southern climates, solar-control Low-E is placed on Surface #2 to bounce solar heat before it enters the gas cavity.
Why does 1/2" cavity width perform best with argon, but 5/8" perform best with air?
If a gas cavity is too narrow (<3/8"), conduction dominates because heat easily bridges the thin gap. If a cavity is too wide (>5/8" with argon), thermal buoyancy sets up vigorous convective circulation loops that rapidly transport heat across the gap. 1/2" (12.7 mm) is the exact thermodynamic sweet spot minimizing both conduction and convection.
What causes seal failure and fogging in double-pane windows?
Hermetic seals (PIB primary seal + silicone/polysulfide secondary seal) expand and contract with barometric and temperature cycles. When the seal fails, moisture infiltrates the cavity, saturating the molecular sieve desiccant beads inside the spacer and condensing permanent mineral fog between the glass panes.